1. Field of the Invention
The present invention relates generally to magnetic devices for safely suspending or levitating movable elements such as are intended for rotation about a predetermined axis, for example, a control moment gyroscope or a reaction wheel. More particularly, the invention relates to protective means for preventing damage to or destruction of such magnetic suspension devices when in operation and of the mechanism or craft supporting the magnetically suspended device.
2. Description of the Prior Art
While magnetic bearings or suspensions for spinning elements such as gyroscopic rotors are widely described in prior patents and in the other literature, typical prior art with respect to the present invention is believed to be represented by the U.S. patent application Ser. No. 433,431, filed Jan. 14, 1974, now U.S. Pat. No. 4,090,745, for a "Magnetic Suspension Apparatus with Magnetic Stiffness Augmentation" in the names of J. R. Dohogne and A. V. Sabnis issued as U.S. Pat. No. 4,090,745, May 23, 1978 and by the U.S. Pat. No. 3,976,339 for a "Magnetic Suspension Apparatus", issued Aug. 24, 1976 in the name of A. V. Sabnis, both assigned to Sperry Rand Corporation. These references will be discussed in further detail within the following material. In general, both reference relate to a magnetic bearing assembly for levitating a movable member either in rotation or in lineal translation and comprising a triple-loop magnetic circuit in which a permanent magnet establishes a quiescent or fixed flux across four magnetic gaps arranged to provide passive restoring forces transverse to the direction of the gap flux. The resulting inherent instability of the suspended element in the longitudinal or axial direction is overcome by an active electric feed back control system which supplies control currents in a pair of solenoids associated with the magnet gaps for modulating the permanent magnet flux therein, thereby providing stabilizing longitudinal or axial magnetic forces. In the event of failure of the magnetic support, or in the event that a severe shock load is accidentally imposed upon the suspending or suspended system, a back-up mechanical axially-disposed bearing system is provided which comes into play when such disturbances occur and for supporting the rotor when the active suspension system is not energized. Thus, only when such conditions are experienced, the rotating mass will be supported by a conventional ball bearing system, normally preventing destruction of the suspension system or associated suspended equipment.
While the configurations described in the aforementioned references perform successfully in many situations, additional problems arise in the operation of such suspensions, particularly those designed for very heavy rotors. The conditions at lift-off and in possibly oscillatory modes may be increasingly severe. High gain and wide band width servomechanisms are required for such rotors, ensuring the possibility of the excitation of violent non-linear oscillations. The more complex rotor systems are susceptible of many mechanical resonances which may couple to the input of the control system, so that the magnetic suspension system is susceptible of undesired oscillations because of the high servo loop gain and servo amplifier saturation effects. Impacting of the back-up bearing during initial suspension and during such oscillatory modes may damage the back-up bearings or produce damaging shocks to apparatus associated with the rotor system, for example, when the rotor is used to stabilize a space satellite.